1. Field of Invention
This invention relates generally to the field of seismic data interpretation. In particular the invention relates to a machine process for selection of three-dimensional (3D) seismic data to provide petroleum exploration professionals more detailed understanding of subsurface geology and geometry. Still more particularly, this invention relates to apparatus and a method by which an explorationist may evaluate the performance of an automated method of "picking" or "tracking" individual seismic events through a three-dimensional volume of data.
2. Description of the Prior Art
FIGS. 1 through 6 of the drawings illustrate features and methods associated with the prior art picking methods; FIGS. 7 through 10 illustrate features and methods of the invention. Only figures associated with prior art methods are introduced here.
FIG. 1 illustrates a portion of a hypothetical 3D seismic data volume in order to explain the three-dimensional relationships discussed in the text and accompanying drawings in this specification;
FIG. 2 is an isometric view of a portion of five seismic traces which illustrates the relationship between a "seed point" and its four adjacent "target" traces;
FIG. 3 illustrates a prior art "simple" or non-iterative automatic tracking method;
FIG. 4 illustrates an example of how a "simple" picking mode or method may fail to pick a target trace;
FIG. 5 illustrates a prior art "iterative" autotracking method.
FIG. 6 illustrates an example of how an "iterative" picking mode or method may fail to pick a target trace.
FIG. 1 is an isometric view of a portion of a hypothetical three-dimensional (3D) seismic data volume. The circles at the top of the volume represent the surface location of individual traces. The vertical lines represent seismic traces which are measured in time or distance along the z-axis of the volume. Each individual trace is an amplitude versus time representation of an acoustic reflection from strata in the earth. A sequence of x versus time traces is called a "line" by seismic explorationists. A sequence of y versus time traces is called a "cross-line". Of course, the y versus time traces may be designated a "line" and the x versus time traces called a "cross-line".
In the seismic art vocabulary, a horizontal section or time slice is a horizontal slice or plane through the 3D volume of data. A "horizon slice" is a slice along or parallel to a structurally interpreted horizon and hence along one bedding plane. Plotting of common amplitudes on x-y axes is similar to a surface topographic map, but of course such a plot is of subsurface strata. Plotting of such different amplitudes at a common subsurface depth illustrates different strata at common depths.
In less than ten years, computer aided exploration revolutionized seismic exploration and field development. Until recently, however, one aspect of seismic interpretation-picking subsurface horizons, or simply, "picking", remained essentially unchanged from paper and pencil method to automatic computer picking methods. A horizon is a surface along a bedding plane of a subsurface formation.
Traditionally, picking was done manually by drawing with colored pencils on paper, one seismic section or line at a time, an incredibly tedious process. In the early 1980's interactive CAEX (an acronym for Computer Aided Exploration) workstations gave seismic explorationists the ability to pick 3D data more quickly and effectively. While interpreting seismic lines (that is, a two-dimensional vertical slice or a "vertical seismic section") was still accomplished by viewing and picking one line at a time, it could then be done by using a mouse in combination with a display screen and clicking the cursor on a few selected points along a horizon and letting the machine pick all the rest of the points on that line. This was the first type of automated picking, and represented an incremental increase in both productivity and accuracy over manual picking.
A horizon is typically displayed on a CRT screen of a workstation, that is, a computer. The display is usually an x-y display including a seed point or points and the "picked" points through the 3D seismic data. The difference in depth or time of the target points from the seed point is indicated, for example, by the color of the picked point.
In one prior art automatic system for tracking a bedding plane or horizon in a generally horizontal zone of 3D data, a user selected or "input" at least one "seed point", which then "expanded" in all four directions within the 3D data volume as illustrated in FIG. 2 until it reached the boundaries of a user specified zone. Users had the option of tracking seismic data in one of two modes: simple (non-iterative) or iterative.
A "seed point" is specified by its x and y location and its time of depth (i.e., the z-axis of FIG. 1). It is also specified by a characteristic of the reflection wavelet at that point. Such characteristic is usually the maximum amplitude of the reflection wavelet at that location in the volume of the data. Other characteristics, such as minimum amplitude, phase, frequency, etc., of the reflection at the x,y,z point may be used. As illustrated in FIG. 3 a first mode is for non-iterative tracking which searches the seismic traces adjacent seed points for similar amplitude values, pick the best one, and then proceeds to the next available trace without double-checking the accuracy of the pick.
FIG. 4 illustrates an example as to how an adjacent wavelet may not be picked in the non-iterative mode. If a negative amplitude is sensed on an adjacent trace at the same time or depth, then such target trace is not selected, that is, it is dead.
A second or an iterative picking mode verifies an adjacent trace as a pick by cross-referencing the previous trace. Once verified, the adjacent trace is treated as a seed point and the picking of adjacent traces from it proceeds. FIG. 5 illustrates such prior art iterative picking. Verification means that if the amplitude of the picked trace is within the limits of tolerance set by the user, the pick is accepted. Users can specify (on a scale of 1-10) the degree of amplitude similarity they are willing to allow. If a pick does not pass this acceptance test, it is designated "dead" until at least one directly adjacent trace matches sufficiently to accept it.
More specifically, once a seed point is selected on a trace, the trace is scanned up and down the z or time axis to find the local extrema amplitude or simply "extrema". A local extremum of a variable x.sub.i where is a digitizing index, is defined as EQU X.sub.i-1 &lt;X.sub.i .gtoreq.X.sub.i+1, or EQU X.sub.i-1 .times.X.sub.i .ltoreq.X.sub.i+1.
Such scanning is bounded by zero crossings of the amplitude of the trace in the case of a peak or a trough. Such extrema will typically vary with time a small amount. For example, if T.sub.0 represents the seed point, T.sub.1 would typically represent the time of the extrema. Next, the time T.sub.0 is started on the target trace. On it, the time is varied up and down between zero crossings of its trace amplitude until the nearest extrema T.sub.2 is found. Finally, the time T.sub.2 is used on the trace on which the seed point exists and on such "seed" trace scanning up and down the "z" axis is again performed for the nearest extrema T.sub.3. If T.sub.3 equals T.sub.1, then iterative tracking has been achieved and tracking continues.
FIG. 6 illustrates an example as to how an adjacent wavelet may not be picked in the iterative mode. Notice that the time T.sub.3 is beyond the zero crossing window of the seed point T.sub.0. Thus, the target trace is not picked.
The amplitude acceptance test tolerance of the prior art iterative tracking mode defines a function, ##EQU1## A.sub.2 = Amplitude of the target wavelet of the target trace at T.sub.2, and A.sub.1 =Amplitude of the seed wavelet from the seed trace at T.sub.1.
The value of S is bounded by values of 0 and 1. The more similar the two amplitudes, the closer the S function is to zero. The more dissimilar the two amplitudes, the closer the S function is to 1. Next, a score function is evaluated: EQU SCORE = (S .times. 9.0)+1.
The score is compared with a control value from 1 to 10 selected by the interpreter or user of the data. Scores greater than the control value prevent a target trace from being picked.
After the tracking proceeds with each selected target trace becoming a seed trace for selecting more target traces, a "horizon" has been picked. The x, y and t (or z) coordinates of each selected wavelet are stored in the computer memory. A horizon is typically displayed on a CRT screen of a work station (a powerful computer adapted for specialized uses such as seismic data interpretation, CAD/CAM work etc.). The display is usually an x-y display of the seed point (or points) and all the picked points which correspond to the seed point of the 3D seismic data. The variation in depth (that is, time) from the seed point is indicated, for example, by the color of the picked point.
The prior art of horizon picking apparatus and method has provided no means by which a user of such apparatus and methods (that is an explorationist such as a geophysist) may determine how many of the non picked points were due to the particular method of picking (e.g. as illustrated in FIGS. 4 or 6) or due to the failure of the apparatus and method to select a target trace in the iterative mode.